title: Effect of variation in the energy spectrum of a cyclotron-produced fast neutron beam in a phantom relevant to its application in radiotherapy.
abstract: The fast neutron spectrum of the beam produced by the MRC cyclotron at the Hammersmith Hospital has been measured in air and at several depths in a water phantom using three field sizes. The neutron spectra were determined both by a set of six threshold detectors and by a liquid scintillator spectrometer. Where a direct comparison of the two methods was possible the agreement was satisfactory. It was found that the shape of the spectrum, above 3 MeV, is unchanged with depth in the phantom, but for each field size the neutron fluence between 0.5 and 3.5 MeV increases significantly and reaches a maximum and the mean energy a minimum. This minimum decreases with increase in field size. Values of the kerma ratio in ICRU muscle to both that in A--150 plastic and bone were calculated and found to vary by less than 0.5% with depth in the phantom: that of carbon to ICRU muscle varied by 5%, and values of W for methane-based tissue-equivalent gas were found to change by less than 1%. Variations in biological response with changes in neutron spectrum are also discussed, together with the clinical significance of the results.
mesh_terms: D005214:Fast Neutrons; D008348:Manikins; D009502:Neutrons; D010315:Particle Accelerators; D011874:Radiometry / Q000379:methods; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy; D012588:Scintillation Counting